The present invention concerns Interpenetrating Polymer Networks (IPNs) containing an epoxy resin having two terminal epoxide groups and a third reactive non-epoxide group. Such IPNs have good tensile strength, fracture toughness, moisture resistance, and thermal stability and are suitable for use as structural material for hot/wet environments, especially for oil field applications.
Epoxy resins can be toughened with rubbers or thermoplastics, but each method has drawbacks. Rubber incorporation significantly improves the room temperature fracture toughness of brittle epoxies at the expense of strength, modulus, and thermal stability. The addition of thermoplastics results in a lesser degree of toughening than rubber modification, but affords better damage tolerance at an elevated temperature.
Currently, the potential methods for the simultaneous enhancement of strength and toughness, without sacrificing thermal stability, are: incorporation of rigid thermoplastic segments into thermosets; addition, via functionalization of pendant groups or addition of functionalized block or graft copolymers, of noncovalent intermolecular forces, for example, hydrogen or ionic bonding; and formation of IPNs for synergistic property advancement.
IPNs are known in the art as unique blends of crosslinked polymers containing essentially no covalent bonds, or grafts between them. [Adv. Chem. Ser., 211 (multicomponent Polymer Materials) Two-and Three Component Interpenetrating Polymer Networks, Klempner et al., Ch. 13, 1986.] A major problem encountered in their preparation is phase separation. The known thermodynamic immiscibility of polymers leads to a multiphase morphology upon normal mixing or blending, hence, leading to certain degrees of phase separation depending on the extent of miscibility.
The art of making an IPN relies on the ability to overcome the thermodynamic driving force for phase separation (i.e., when one of the components forms a discrete phase while the other forms a continuous phase). The occurrence of phase separation in IPNs leads to undesired structural inhomogeneity and defects, and consequently poor mechanical properties.
Conventional IPN technology avoids phase separation of constituent polymers by introducing physical entanglement between two polymers. This is done by polymerizing or crosslinking the two polymers sequentially or simultaneously. Sequential synthesis involves the swelling of polymer A with monomer B and a crosslinking agent; B is then polymerized and crosslinked. In simultaneous synthesis, monomers A and B are polymerized and crosslinked by noninterfering mechanisms. (Encyclopedia of Polymer Science and Engineering, 2nd edition, V. 8, 279, 1986.) The present invention alleviates phase separation and also forms internetwork bonds between two crosslinked networks.
Methods for increasing the toughness and strength, without sacrificing thermal stability, of polymers by IPN technology are known in the art. Normally, such IPNs consist of a rigid polymer network for strength, and a ductile, but high Tg, polymer for toughness. The Langley Research Center reported in "Tough, Microcracking-Resistant, High-Temperature Polymer," NASA Tech Briefs, June 1990, pg. 64, the simultaneous synthesis of a semi-IPN (i.e., one of the polymers is crosslinked while the other is not) polyimide consisting of a thermosetting polyimide and a thermoplastic polyimide. Toughness, resistance to microcracking, and glass transition temperature were all improved. Typically these new compositions are used in the aircraft/aerospace field and some are being considered for a variety of electrical and electronic applications.
U.S. Pat. No. 4,468,485 teaches the preparation of IPNs such as Epoxy/unsaturated polyester with one polymer (unsaturated polyester) crosslinked by radical reaction while the other (epoxy) is crosslinked by heat-activated polyaddition. No chemical bond exists between the two polymers.
ICI's EP 311,349 concerns thermoplastic polymer/thermoset polymer IPNs, for example, an amino-terminated polyarylsulfone (a thermoplastic) and an epoxy resin. In this case, the cocontinuous phase morphology, demonstrating the best mechanical properties, was obtained only at a certain range of composition ratio between the thermoplastic and thermoset polymers. Such an IPN resulted in a single glass transition temperature indicating molecular level miscibility. This is the case of a semi-IPN, since the thermoplastic phase itself does not form a network.
Akzo's EP 417,837 describes the formation of a "chemically-linked" epoxy/triallylcyanurate IPN, in which the triallylcyanurate is crosslinked by radical initiator while the epoxy is crosslinked by a curing agent that also contains a radical-crosslinkable double bond. Such curing agent al so serves as the chemical link between two polymers. Conventional IPNs have no chemical bonding between network polymers, rather, the polymers are physically entangled. One single Tg and good thermal stability was claimed. One aspect of the present invention uses diamine as the crosslinker, which is known to inhibit radical polymerization, and therefore precludes the use of free radical reactions to form chemical bonds between the two IPN networks.
Dow Chemical's U.S. Pat. No. 4,594,291, issued to Bertram et al., teaches the preparation of relatively high molecular weight epoxy resins by prereacting regular epoxy resins with a curing agent. Such an epoxy/curing agent mixture was shown to provide improved toughness and processability compared with mixing of the standard low molecular weight epoxy resin and a curing agent. One specific example disclosed is the use of sulfonic acid amide as the curing agent for prereaction. Such curing agent is exemplified by the formula ##STR1## where R is a hydrocarbon group having from 1 to about 4 carbon atoms. Two active hydrogens on one of its amino end groups will link into part of the epoxy backbone while the other two, due to their relatively lower reactivity, are available for further reaction with epoxies at elevated temperature. Such a resin alone has a viscosity too high for conventional composite processing technology.
Epoxy IPNs containing a rubbery network are also known in the art. For example, Epoxy/butyl acrylate IPNs were synthesized at Lehigh University, Scarito and Sperling, Polymer Engineering and Science, 19, 297 (1979). The resulting IPN exhibits two phase morphology, improved toughness, but lower tensile strength. Glycidyl methacrylate, with one epoxy and one polymerizable acrylate, was added to form chemical grafts between epoxy and butyl acrylate networks. The appearance of two Tgs and phase separation indicates that the formation of chemical grafts by radical copolymerization with a bifunctional comonomer is not sufficient to suppress phase separation between two polymers.
Epoxy/PU graft-IPNs were prepared by Frisch et al. [H.L. Frisch, K.C. Frisch and D. Klempner, Polym. Eng. Sci., 14, 646 (1974)], using excess isocynates, the chain extender for PU, to form covalent links with the pendant hydroxyl groups of epoxy polymers. The resulting material had improved tensile strength and elongation (232%) and showed one glass transition temperature at about 70.degree. C. However, such a material is too flexible for structural applications.
The limitations of current epoxy polymers as the matrix in polymer composites for use as structural materials in corrosive oil field applications are known. Oil field pipes require long-term resistance to medium pressure CO.sub.2 and H.sub.2 S gases (ca. 150 psi) and 5% salt water at 150.degree. F. This obviates the need for reversible tensile behavior or a composite yield strength of about 55 ksi. Existing polymer composites fail to meet the 55 ksi requirement because they exhibit weeping failure {the diffusion of fluid molecules through the thickness of the pipe) due to yielding below 20 ksi. This weeping failure is preceded by matrix microcracking, which in turn is attributable to insufficient matrix toughness. The present invention overcomes these limitations by improving matrix toughness.